Airscrew



y 1949- H. 'r. AVERY 2,475,121

AIRSCREW Filed Oct 9, 1943 7 Sheets-Sheet 1 MMM ATTORNEYS.

H. T. AVERY July 5, 1949.

AIRSCREW 7 Sheefs-Sheet 2 Filed Oct. 9, 1943 INVENTOR: f/qro/a [Avery BMM ATTORNEYS.

H. T. AVERY July 5, 1949.

AIRSCREW 7 Sheets-Sheet 3 Filed Oct. 9, 1943 INVENTOR: Harv/a Z AveryATTORNEYS.

July 5,1949. H. 'r. AVERY unscnmw I Filed Oct. 9. 1943 7 Sheets-Sheet 43! B A a 1/100 .3!

f0 4 5 11 L i f f y ffl 4 5% I O I I I, 49 44 I F IE E INVENTOR:flaro/dffll/erg ATTORNEYS.

H. T. AVERY July 5, 1949,

AIRSCREW 7 SheetsSheet 6 Filed Oct. 9, 1943 FIE ZLE ATTORNEYS.

y 949. H. 'r. AVERY 2,475,121

I AIRSCREW Filed Oct. 9, 1943- 7 Sheets-Sheet 7 INVENTOR: flar'a/aTAU/erg ATTORNEYS.

Patented July 5, 1949 UNITED STATES PATENT OFFICE AIRSCREW Harold T.Avery, Oakland, Calif.

Application October 9, 1943, Serial No. 505,636

10 Claims. 1

The present invention relates to the improvement of airscrews foraircraft, and more particularly to the design and/or adjustment of theshape of the airfoil elements thereof for the purpose of enhancing theefiectiveness and efiiciency of airscrews under widely varyingconditions of flight.

Conventional airscrews employed as propellers are designed to attain themaximum efliciency of which they are capable only at approximately thecruising speed of the craft, and they are relatively ineincient undertake-off conditions where the static-thrust eiiiciency of the airscrewand its efliciency at low rates of axial advance are controlling.Airscrews employed as sustaining rotors for helicopters or helicogyrosoperate at all times under conditions in which the static-thrustefiiciency of the airscrew and its efficiency at low rates of axialadvance are controlling, and therefore when they embody the samecharacteristics of design which have rendered propelling airscrewsineflicient under such conditions of operation the sustaining rotors aresimilarly inefi'icient.

I have concluded that this decrease of efficiency under conditions ofstatic-thrust and low rate of axial advance is due, at least in a largepart, to radial movements of air in the slip-stream of the airscrew. Itis the present practice so to shape the blades of airscrews that thevelocity increment imparted to the airstream thereby is largely effectedby the outer portion of each blade. It follows that the velocity ofdifferent radial portions of the slip-stream is then distinctly unequaland dissipation of energy will necessarily occur by reason of the radialcirculation which develops to minimize these inequalities.

At high craft velocities the velocity increment imparted to the air by apropeller is relatively low, and dissipation of energy by such radialcirculation in the slip-stream may be negligible. However, underconditions of static thrust and low rate of axial advance the velocityincrement imparted to the air by either a propeller or a sustainingrotor is relatively high, and the energy loss arising from such radialcirculation in the slip-stream is magnified.

Having in mind the foregoing deficiencies of existing airscrews foraircraft, I have discovered that the eiflciency as well as the thrustper unit of airscrew disc area capable of being exerted by any airscrewcan be enhanced by a novel combination of shape-and-pitch adjustingmeans with a preferred form of blade; either the adjusting means or theblade form being capable of producing such results to a minor degreewhen employed either alone or in conjunction with known blade shapes orpitch adjusting means, respectively, but the two in combinationenhancing efficiency and thrust per unitof airscrew disc area to anextent exceeding the sum of the percentages of increase in these figuresof merit ob tainable by the use of either in conjunction with knownblade shapes or pitch adjusting means, respectively.

In general the shape-and-pitch adjusting means of my invention isdistinguished by the fact that it is capable of varying the pitch ofdifferent portions of each blade by different amounts, respectively,producing a change in the twist of the blade at the same time that thepitch of each portion is changed. This adjusting means is such that whenthe airscrew is stationary or moving axially with respect to the air itacts to effect a greater change of pitch near the root of the blade thannear its tip, and ideally the pattern of this pitch change should besuch that the tangent of the pitch angle change at every point along theblade is inversely proportional to the radius of each such point fromthe axis of the airscrew.

As soon as any lateral movement of the airscrew relative to the air isintroduced, as in the forward flight of a helicopter, the desirablepattern of pitch distribution changes, and becomes a function of manyfactors including the relative rates of axial and lateral movement ofthe airscrew relative to the air, and of the rotational tip speed of theairscrew blades. The shape-andpitch adjusting means of my invention isfurther distinguished by the fact that it is capable of altering thepattern of pitch distribution in conformity with such factors.

While certain advantages can be realized by employing my novelshape-and-pitch adjusting means in conjunction with airscrew blades ofconventional shape, I have discovered that the maximum advantage can berealized only by using this means in combination with airscrew blades soproportioned that the blade width varies substantially in inverse ratioto the radius from the axis of the airscrew. Blades proportioned in thismanner will have large root portions which I have discovered should beblended into a central core of correspondingly large diameter so as toeliminate from the active airstream that portion of the airscrew arealying too close to center to permit of proportioning the bladessubstantially in the above mentioned ratio. The prime objectives to beattained or at least approached are: first, the creation of a slipstreamof uniform, axial velocity throughout the active airscrew area; and,second, the maintenance of such uniformity at all pitches of the blades.

When my novel shape-and-pitch adjusting means is employed in conjunctionwith blades of conventional shape which, as pointed out hereinbefore, donot attain the first of these objectives, the only advantages realizedare those arising from the avoidance of the introduction of stillfurther inequalities of slip-stream velocity upon changes of pitch.

When airscrew blades so proportioned that the blade width variessubstantially in an inverse ratio to the radius from the axis of theairscrew are employed in conjunction with conventional pitch adjustingmeans incapable of altering the twist of the blade, the first of theabove stated objectives can be attained at only a single pitch setting;because to vary blade pitch and yet retain such a pattern of pitchdistribution as will make it possible to produce a slip-stream ofuniform axial velocity throughout the airscrew area, requires a greaterchange of pitch near the root of the blade than near its tip, i. e., achange in the twist of the blade.

However, airscrew blades so proportioned that the blade width variessubstantially in an inverse ratio to the radius from the axis of theairscrew if employed in conjunction with a specifically correct pitchdistribution are capable of attaining the first of the objectives abovestated, and when my novel shape-and-pitch adjusting means is employed incombination with such blades, the second of the above stated objectivesis also attainable, thus making possible not only a marked increase inmaximum airscrew efficiency, but the maintenance of that efficiency overa wide range of operating conditions.

It is therefore a primary object of my invention to provide an airscrewso shaped and proportioned as to more effectively and eflicientlyproduce thrust.

It is, more particularly, an object to provide an airscrew capable offunctioning at high efficiency over a wide variety of operatingconditions, including particularly conditions involving a low rate ofaxial advance of the airscrew.

It is a further object to provide superior means for adjusting anairscrew to facilitate efficient performance under such variedconditions.

It is a further object to provide means for automatically altering thepatterns of adjustment of an airscrew to better adapt it to variousconditions of operation.

It is a further object to provide improved construction for airscrewblades having especial advantages in facilitating the manufacture ofblades of the improved shape and characteristics.

It is also an object to provide airscrew hub constructtion both improvedmechanically, particularly with regard to the method of mounting andcontrolling the blades, and improved aerodynamically both in theexternal shape of the hub unit itself and, particularly in the case of asustaining rotor, in the shape of its junction with the aircraftfuselage.

More specifically it is an object to provide improved means for changingairscrew blade pitch, including means for altering the pattern of pitchdistribution along the blade to best adapt it to difierent operatingconditions.

It is also an object to provide especially simple and effective meansfor selectively adjusting the pitch of airscrew blades.

Additional objects and advantages of the invention will be apparent inthe course of the following description of preferred embodiments thereofwhich is to be read with reference to the accompanying drawings, inwhich:

Figure 1 is a plan view of a rotating wing aircraft having a sustainingrotor constructed in accordance with my invention;

Figure 2 is a front elevation of the same craft;

Figure 3 is a perspective view of the framework and pitch adjustingmechanism for one blade of the rotor of the craft illustrated in Figures1 and 2, and of the mountings and controls for all blades thereof;

Figure 4 is a cross section of a rotor blade taken on line 4-4 of Figure1;

Figure 5 is a vertical section through the rotor hub, takensubstantially on line 5-5 of Figure 6, showing the main rotor driveshaft, and the mechanism for controlling the blades;

Figure 6 is an enlarged plan view of the rotor hub mechanism and themounting and pitch adjusting mechanism for one blade, shown partly insection;

Figure '7 is a diagram showing typical pitch adjustment patterns for arotor blade constructed in accordance with my invention;

Figure 8 is a diagrammatic view showing in elevation various possibledisplaced positions of blade center line and reference lines;

Figure 9 is a plan view similar to Figure 6, but illustrating analternative blade construction arrangement and showing the mountingarrangement for one blade only;

Figure 10 is a front elevation of a propeller, adapted for airplanepropulsion, constructed in accordance with my invention;

Figure 11 is a section through the propeller of Figure 10 taken on lineII--Il of Figure 10, showing particularly the blade construction andblade pitch adjusting means;

Figure 2 is a cross-section of a blade of the propeller of Figure 10taken on line I2-l2 of Figure 11, and

Figure 13 is a front elevation of another embodiment of my invention,showing its application to a ducted propeller.

Figure 1 illustrates a helicopter having a sustaining rotor l0 embodyingthe preferred form of my invention, in accordance with which each bladel I is so tapered that the blade width varies substantially inversely asthe distance from the central axis [2 of the rotor. As illustrated thisrelationship continues inwardly until the outlines of adjoining bladesintersect or coincide with each other. As illustrated also in Figure 2the rotor is joined to the fuselage by a streamlined enclosure l3 ofapproximately as large a diameter as can be inscribed within the centralsolid area formed by the merging of the roots of the blades asillustrated in Figure 1. This central area extending outwardly as far asjunction lines IS on the respective blades is covered with a streamlinedcovering IG of material which is highly flexible but stiff enough toreasonably retain its shape, such as, for instance, the rubber andfabric composite material of which the casings for automobile tires arecustomarily built.

The internal construction of the rotor is illustrated in Figures 3 and4. The main framework of each blade consists of a channel [8, preferablyformed of strip metal, and hence easily constructed to conform to thetaper of the blade. Channel I8 is reinforced and stiffened by at leasttwo plates I9 and 20 which extend squarely across the blade and by aplurality of diagonal plates 2!, all of which plates are welded orotherwise integrally attached to the channel. Integrally attached toplate 20 and extending inwardly therefrom is a shaft 23, preferably inthe shape of a hollow metal tube. Shaft 23 passes freely throughapertures in all plates 2i lying between plates I9 and 20. In some ofthe plates the shaft passes through a clearance hole 24 without touchingthe plate, while bearings (not shown) may be integral with certain ofthe plates M to pivotally support shaft 23. Shaft 23 is also pivotallysupported in plate I9 by a gear 25 integral with plate I9 and serving asa bearing for the shaft as shown in Figure 6.

Since plate 28 is integral with shaft 23, any rocking of plate I9 onshaft 23 will result in a torsional deflection of the portion of channelI8 extending from plate I9 to plate 20. Brace plates M are so locatedthroughout this portion of the channel (or otherwise so dimensioned) asto vary the torsional stiffness thereof in accordance with such aprearranged pattern that any given angular displacement of plate I9relative to plate 23 will cause the intermediate sections of the channelto assume angular displacements arranged in accordance with aprearranged pattern, preferably one in which the amount of angulardeflection of the channel within each section thereof is substantiallyproportional to the difference of the reciprocals of the radii from axisE2 to the two ends of the section. By greater or less spacing of braces2! the torsional stiffness of the various portions of the channelframework may be altered in any desired manner, so that the arrangementfor braces 2I actually adopted will provide a distribution of torsionalstiffness varying in accordance with the desired pattern. The outerportion of the channel beyond plate 20 may be arranged to constitute asubstantially rigid framework, at least so far as torsional deflectionis concerned, because it is relatively unimportant to alter the twist ofthe blade in the portion adjacent to its tip; but between plates I9 and20 the torsional stiffness is low enough so that considerable angles oftwist may readily be introduced.

Plate l9 (Figures 3 and 6) on each blade is preferably located atapproximately the vicinity of junction line I5 (Figure 1) at which linethe nature of the exterior covering changes. From line I5 out, eachblade is covered with an outside covering 21 (Figure 4) adapted topermit the torsional deflection above mentioned, which covering may beof treated fabric or very light sheet metal or a combination of the two.To support this covering 2'! in its proper contour, auxiliary frameworkincluding brackets 28, 29, and 3!! (Figures 3 and 4) may extendintegrally out from channel I8.

The method of attachment of the blade to the hub of the rotor and ofcontrolling its pitch is shown in Figure 3, and in greater detail inFigure 6. Integral with the main vertical driving shaft 33 of the rotoris a hub member 34 to which is hinged as at 34a and 34b a link 35 foreach blade; the aXes of these hinges extending along a flapping hingeline 36, the forward end 36a of which slants upwardly and outwardly.Each link 35 is hinged to a blade root swivel 38 by means of drag hinge39 perpendicular to the blade axis ill but inclined about 45 to the ver-6 tical. Shaft 23 is locked into swivel 38 by means of a thrust bearing,which however leaves the shaft free to turn in the swivel upon its oWnaxis I0. Integral with swivel 38 is a flange 40 carrying a stud 4| onwhich is rotatably mounted a compound gear 42, 43. The smaller gear 42meshes with a gear 44 keyed to the shaft 23, While the larger gear 43meshes with a gear 25 integral with the plate I9.

As is well known in connection with rotating wing aircraft each blade ispositioned about its flapping and drag hinges by the forces acting onthe blade, including particularly aerodynamic and centrifugal forces.For each such position of the blade the position of flange 40 is fixed,and therefore the position of stud 4| is fixed relative to shaft 23, butplate I9 is subject to angular displacement on shaft 23 by virtue ofcontrols to be presently described. Through the action of the gear train25, 43, 42, 44 this will result in shaft 23 being rotated in the samedirection as plate I9, but through a smaller angle constituting a fixedproportion of the angle through which plate I9 is rocked. With the gearratios illustrated in Figure 6 shaft 23 will rock through approximatelyone-fifth the angle that plate I9 rocks through, but the most desirableratio of angles in any given embodiment will depend upon a number offactors to be discussed hereinafter.

The effective blade pitch is controlled by rocking plate I9 about shaft23, this being effected by alterin the Vertical position of the innerend of a rod 45 integrally attached to an inwardly extending arm 4'! ofchannel I8. Attachment of rod 46 to arm 41 is preferably made by twobolts 49 which extend through holes in rod 46 and may be placed in anyof a series of holes I3 in arm 41 to permit alteration of the effectivelength of the connection for a purpose hereinafter described. The innerend of rod 46 is connected by ball and socket joint 50 to a link 52which extends up approximately vertically from a swash-plate spider 53,to which it is attached by means of ball and socket joint 55 (Figures 3and 5). Through means described hereinafter spider 53 may be raised,lowered, or tilted in any direction thus at any instant raising orlowering a given ball and socket joint 50 and thereby altering theangular settin of plate I9 upon shaft 23 as a pivot. Any change of pitchtransmitted to plate I9 through the raising or lowering of joint 50 andhence of bar 46 will be proportionately reduced by the gear train 25,43, 42, 44 and transmitted to shaft 23 in this reduced amount.

Figure '7 shows diagrammatically a number of typical patterns of pitchdistribution along a rotor blade. It will be noted that these have beenshown as pertaining to a blade 18 feet long, measured from rotor axis,and having plate I9 located 3.0 feet from the axis and plate 20 located14.0 feet from the axis. What is shown as 0 pitch angle does notnecessarily mean a blade angle at which advance in the plane of therotor disc would produce no lift, but preferably refers to an arbitrarybasic angle of blade setting capable of producing lift, somewhatcomparable for instance with the usual practice in regard to liftcoefficients of air foils for airplane wings. The pitch angles plottedas ordinates then represent angular increases in pitch setting from thisbasic pitch angle.

When the travel of a helicopter includes a horizontal component ofmovement relative to the air the pattern of air flow through the rotorbecomes relatively complicated, but under all other operating conditionsall elements of flow through the rotor disc are essentially axial exceptfor a rotational component of the airflow which in helicopter rotors isso small as to have only a negligible effect on blade pitch. Thereforewhen, in accordance with the teaching of my invention, substantiallyuniform axial flow over the entire disc is attained, a particularlysimple flow pattern results; the angle of air movement relative to theblade being, at each radius, substantially the angle whose tangent isthe axial velocity divided by the rotor blade rotational velocity andtherefore being such that the tangent of the angle varies inversely withthe radius.

In Figure '7 the dotted curve 60 is a typical one following thispattern, wherein the tangents of the angles vary inversely with theradius. The gear train 25, 43, 132, M is not adapted to precisely followthis pattern, but is one in which the angles, rather than the tangentsof the angles, are proportional. Curve BI is one in which the anglesthemselves vary inversely with the radius, and it will be noted that thetwo curves are so nearly identical that the difference will notsubstantially afiect the result obtained. If, as illustrated, plate 20is at three-fourteenths the radius of plate l9, and if, as in the caseof curves 60 and iii, the pitch angle at plate [9 is 00 the pitch angleat plate will be 317 on the basis of tangents varying inversely at theradius (curve 60) and will be 313 on the basis of angles varyinginversely as the radius (curve 6|) showing the difference to be verynegligible. By slight alteration of the pattern of torsional stiffnessthe blade may be made to more nearly follow curve 60 than curve 6| forpoints intermediate between plate l9 and 20. However, the effect of therotational component, which was stated to be negligible, would be torender desirable a little more than proportional steepening of angletoward the root of the blade, and it is therefore considered desirableto follow the pattern of curve 6!], wherein the angles are inverselyproportional to the radii, or, in case of a particularly heavy rotordisc loading, a curve slightly more concave than curve 6| together withpossibly a slightly higher ratio of gearing down between the movementsof plate l9 and plate 20.

Since the arrangement as illustrated in the drawings and previouslydescribed does not contemplate introducing any twist into the bladebeyond plate 20, the changes of pitch will be substantially constantbeyond plate 20. Thus, if that portion of the blade is constructed inaccordance with the portion of curve 6| lying between point 62 and point63, the only type of pitch change that can be made for this portion ofthe blade will be equivalent to a bodily raising or lowering of thisportion of curve BI, and it will be noted that the only other solid linecurves in Figure '7 for this portion of the blade accord with this.

If pitch is now to be increased to twice as steep a setting as that ofcurve 6|, dotted line curve 65, Figure '7, results, wherein each angleis exactly twice that of the corresponding angle for curve 6|. However,to follow this curve perfectly would require warping of the blade clearout to the outer tip, and while this would be possible the particularembodiment disclosed does not contemplate doing this. Curve 66illustrates how closely curve 65 may be approximated by an actualadjustment of the blade without warping the outer four feet thereof. Itwill be noted that the angular displacement of plate 20 is shown asaccording with that theoretically required for the mid-point of theouter unwarped section, namely for 16 foot radius, instead of beingrelated to that of plate [9 exactly in the inverse ratio of therespective radii to the two plates, meaning that plate 20 is displaced Winstead of "714 the amount that plate [9 is displaced. In case ofreduction of pitch setting the blade will then, for instance, assume thepattern shown by solid line curve 68, which, it will be observed,coincides very closely with the theoretical curve 69.

The ratio of reduction of angular movement effected by the gear train25, 43, 42, 44 is not, in general, made equal to the desired ratio ofthe angular displacements of plates l9 and 20, but will in general besuch as to effect a still further gearing down to allow for thetorsional deflection of shaft 23 under the loading which produces thedesired torsional deflection of the blade structure. The required gearratio may be determined as follows:

Let g=the required ratio of angular displacement of gear 44 to that ofgear 25 Let A1=a desired change in pitch angle setting of plate 20 LetAz=corresponding change in pitch angle setting of plate I9 LetB1=corresponding change in angular setting of inner end of shaft 23 Letri=radius to plate 20 Let r2=radius to plate l9 Let R=radius to tip ofblade Let f1=torsional deflection of shaft 23 under unit load, which isinversely proportional to the stiffness of shaft 23 Let fzztorsionaldeflection of blade frame from plate l9 to plate 20 under same load,which is inversely proportional to the stifiness of this portion of theblade frame.

Since the actual torsional deflections of the blade frame and of shaft23 at a given blade pitch setting will bear the same ratio to each otheras that of the torsional deflections of these two members under unitload From this it will be seen that the gear ratio will be in the ratioof the desired angular displacements of plates 19 and 20 only if thesecond term disappears, which will be true if f1 equals zero, whichwould mean using a shaft 23 that is absolutely rigid, or at least hasnegligible torsional deflection under all operating conditions. With anyappreciable torsional deflection of shaft 23 the gear ratio becomessmaller (that is greater gear reduction) than the ratio of the platedeflections.

If the gear train were dispensed with and shaft 23'made integral withrings 31 of hinge 39, as illustrated in Figure 9, it would be equivalentto reducing the gear ratio, 9, to zero, which could be done and theproper ratio of angular displacement of plates l9 and 20 stillmaintained if Equation 4 is satisfied with 9:0, which will be the caseif f1 A1 A1 namely if the torsional stiffness of the shaft and the bladeframe are in the ratio of the desired angular displacements. The Figure9 embodiment should therefore be so constructed that the ratio ofstiffness defined by Equation 5 will maintain.

If the angular displacements are inversely proportional to the radii,with the displacement of plate 20 corresponding to that required for apoint half way between plate 20 and the tip of the blade, Equation 4-.may be rewritten to express the required gear ratio in terms of theradii and Equation 5 rewritten to express the ratio of torsionalstiffness making possible the elimination of the gear reduction asillustrated in Figure 9 as follows:

As previously noted, it is desirable that the portion of the bladebetween plate I9 and plate 20 deflect in such a manner that the pitchangles at various distances from center will be substantially inverselyproportional to the respective distances. To express thismathematically,

The condition stated above is then:

The rate of change of pitch angle at variousradial distances from rotorcenter may then be determined by differentiation, as follows:

dA k equals F A equals However, from the laws of torsional deflection,We know:

From Equations 9 and 10, it is evident that the effective polar momentof inertia must vary sub- Z equals 7 st-antially in direct proportion tothe square of the radius from rotor center if the blade is to undergotorsional deflections which bring it substantially into accord withcurves 6|, 65, and 69 (Figure 7). That is:

Certain of the foregoing equations may be further simplified by letting:

A equals (l2) and f1 f equals (13) f, therefore, represents the ratio ofthe 10 torsional stiffness of the torsionally deflected portion of theblade frame to that of shaft 23.

Inserting these values in Equations 4, 5, 6, and 7, respectively, wehave:

9 equals 11- (l-a) (14) f equals (15) As a specific illustration of theeffect of the stiffness of shaft 23 assume r1=14 feet r2=3.0 feet R=18feet Then if shaft 23 is essentially rigid and f1=0 g=0.1875 fromEquation 6.

If the shaft is not rigid but is ten times as stifl torsionally as theblade frame, that is then g=0.1062, requiring almost twice as great agear reduction as with the rigid shaft.

From Equation 7 we find that to eliminate the gearing and adopt theFigure 9 arrangement it is necessary to establish the stiffness ratio sothat:

For a craft in which the changes of blade pitch angle are not so greatthat this ratio would result in too limber a blade such an arrangementwould constitute a preferred embodiment. For

values of 1'1 less than this, 9 becomes negative, which means that iffor any reason it is desired to utilize a shaft 23 with greatertorsional flexibility relative to the torsional flexibility of the bladethan that which would give the desired deflection pattern with thegearing eliminated, this may be done providing the gear train is soarranged as to produce a displacement of the root of shaft 23 oppositein direction to that of plate l9, as by interposing an additional idler.

When the advance of the sustaining rotor of a rotating wing aircraft isother than axial, as for instance when it is travellin forward at itsnormal cruising speed, the velocity and angle of airflow over each bladevaries cyclically giving rise to a rather complex pattern of airflowthrough the rotor, which pattern changes considerably with changes offlight conditions. The determination of a desirable pattern for pitchdistribution is therefore not as simple nor capable of as definite andgeneral a solution as in the case of axial flow. It is, however, veryclear that the relatively steep pitches, which as outlined above, aredesirable near the roots of the blades under certain conditions of axialflow, are no longer desirable at least in comparable degree in case oftranslational movement. If the direction of rotation of the rotor issuch that a blade on the right side rotates toward the rear of the craft(such a blade being known as a receding blade when the craft isadvancing), and a blade on the left toward the front of the craft (sucha blade being known as an advancing blade), then in normal flight therotor will, at any instant, be revolvin in space about a point locatedto the right of the rotor axis and spaced from it by a fraction of therotor radius corresponding to the tip-speed ratio, namely the ratio ofthe speed of advance of the craft to the tip rotational velocity, whichratio is usually about one-third, but may become as large as two-thirds.

Therefore the movement of an advancing blade relative to the generalbody of air is in case of forward flight generally comparable to that ofthe outer two-thirds or so of a blade under axial flow conditions andthepattern of pitch distribution should preferably accord with that forpatterns previously developed for the outer twothirds or so of theblade, using relatively low effective pitch because of the relativelyhigh velocities, and the desirability of minimizing upward flapping onthe advancing side. On the receding side on the other hand, the blade isrotating in space about a point usually lying within the middle third ofthe blade itself. Therefore, relatively high pitches would be desirablein view of the low air velocities and the desirability of minimizindownward flappng on the receding side, and if possible we should havethe pitches steepening toward the middle of the blade, and if possiblereversing or at least reducing near the root. It does not seempracticable to approximate very closely the varying type of patternoutlined above but the present invention makes it possible more nearlyto do so than with former blade arrangements.

Various arrangements have been utilized in the prior art to decreasepitch on the advancing side and increase it on the receding side. In onearrangement this is accomplished by the angling upward and outward ofthe forward end of the flapping hinge, with the blade so attached to thehinge that its pitch is altered by its angular displacement about thehinge. In another arrangement the blade is pivotally mounted for angulardisplacement about its own longitudinal axis and pitch is controlled bya link extending forward from the blade outboard from the flapping hingeto a pivotal control connection thus enabling flapping to produce thesame pitch changing effect as if the blade pitch were fixed by aflapping hinge passing through the intersection of the blade axis withthe flapping hinge and through the pivotal control connection. Byutilizing both of these arrangements and combining them in a novelmanner I am able to secure a resulting pattern of pitch change inresponse to flapping, which not only differs radically from thatintroduced for controlling vertical movements of the craft so that atype of pattern respectively more favorable for each type of operationmay be secured, but which also may be made to vary throughout eachflapping cycle, thus introducing the possibility of better adapting itto the various phases of the cycle.

The soecific pattern of pitch distribution which the blade will adopt ateach given phase of the cycle under any given flight condition dependsupon the dimensional relationships of the mechanisms employed forintroducing from the two different arrangements mentioned the respectivecomponents of pitch change in res onse to flapping movement. To be morespecific the angle A (Figure 6) between the blade axis and the flappinghinge axis 38 determines the amount of angular displacement of pin 4|about blade axis 10, while the location ofball and socket joint50determines the angular rock of plate l9 about axis 10 in response toflapping, and these two 12 combine to determine the angular rock ofshaft 23. Therefore the relationship of joint 50 to hinge axis 36 andblade axis 10 determines the patterns of pitch distribution effectiveduring flapping cycles.

This relationship may be altered so as to adapt the craft for deliveringits best performance under selected flight conditions. In the embodimentdisclosed such alteration is not contemplated during flight, but a givencraft may be altered to better adapt it to efficient performance underheavier loads, or at higher speeds, or under whatever general type ofservice it may be desired to favor. Such alterations may be effected byremoving bolts 49 and fastening them into other holes 13 of arm 41,whereby joint 50, instead of being located, as illustrated in Figure 6,on a line 14 perpendicular to blade axis 10 at its intersection withhinge axis 36, may be made to occupy a position 50a on the back side ofline 14, or a position 50b, intermediate between line 14 and hinge axis36, or a position 500 on hinge axis 36, or a position 5011 outside hingeaxis 36, or any other position desired. Preferably bolts 15 (Figures 3and 5) may be released and utilized to reclamp spider 53 to its mountingat a new rotational position such as to maintain links 52 substantiallyvertical in each readjusted position of joint 50.

The effect of the location of joint 50 on the pattern of pitchdistribution along a blade may be best understood by reference to Figure8, which shows in elevation various relative positions of certain linesdeterminative of blade pitch. These lines include rotor axis l2, bladeaxis 10, and an arbitrary reference line 11 normally parallel to bladeaxis 10 and fixed in the blade in the vicinity of its leading edge. InFigure 8 blade axis 10 is shown in the position which it occupies whenthe blade is rocked upward about its flapping hinge sufficiently so thatthe blade pitch is reduced to zero or to such other basic value asplaces reference line H in such a position that its projection onto theplane containing axes l2 and 10 coincides with blade axis 10. If blade70 be now rocked downward about its flapping hinge, which in the Figure8 projection coincides with point 50, into the position 10 referenceline 11 will assume a position dependent upon the position of point 50relative to the hinge lines and axes. If, for instance, joint 50 islocated at the position designated 50c in Figures 6 and 8 with spider 53(Figures 3 and 6) adjusted to such a height as to bring the jointexactly on the flapping hinge line 36, then the blade will rock aboutline 36 as an axis without any rolling of the gear train 25, 43, 42, 44and reference line I! will remain parallel to axis 10 at all times,assuming the position when the blade axis is rocked down to position 10.

In analyzing other positions of joint 50 it will be assumed for thepurpose of the analysis that spider 53 (Figures 3 and 6) is brought ineach instance to such an adjustment as will place the projection ofjoint 50 in Figure 8 exactly on line 70. While this will be a differentadjustment of spider 53 in each case it will have the effect of insuringthat at the flapping angle represented by line 10 the blade will in eachinstance have the same basic pitch (nominally zero pitch) and thereforethe positions assumed by reference line 11 upon rocking blade axis 10down to position 10' will be indicative of the difference in effect onblade pitchbroug'ht about by rocking the blade through a fixed angle.The location and configuration that reference line 11 will assume forpositions of joint 58 displaced from hinge axis 35 (Figure 6) can bestbe determined by assuming that the blade axis is first rocked fromposition I8 to position I8 with the gear train 25, 43, 82, 44 locked,and that whatever vertical dis placement of joint 58 that this wouldentail is permitted to take place, which rocking will always bring lineI! to position He, and that thereafter the gear train is freed and joint58 returned to the vertical height it held prior to rocking which meansthat the vertical displacement which would be necessary to move joint 58from line 110 to line 18 will produce a pattern for line 11 accordingexactly with the pattern which would be produced by this same amount ofvertical displacement of joint 58 as previously discussed in connectionwith axial flow conditions and exemplified by curves 68, GI and 66 ofFigure 7. Thus curves. Ha, I1 and 11b and 11d of Figures 7 and 8indicate the patterns assumed by reference line H and hence the patternsof pitch angle distribution effective with the blade axis in position 18and with joint 58 in the positions indicated in Figures 6 and 8 aspositions 58a, 58, 58b, and 58d, respectively. The locations of theintersections of these respective curves with lines I9 and 28 in Figure8 correspond to the respective deflections of plates I9 and 28 inresponse to the above mentioned vertical displacements of joint 58. Itwill be noted that a blade with a high flapping angle may have asubstantially uniform low angle of lift corresponding to line 19 ofFigure 7, while a blade with a low flapping angle may, for the jointposition identified as 58 in Figures 6 and 8, have its pitch increasedin accordance with curve 11 of Figure '7. On the other hand for flightconditions under which it would be more advantageous for the change ofpitch with change of flapping angle to be more uniform throughout thelength of the blade, joint 58 may be located at position 580 and therelatively level pitch curve 110 secured (Figures 7 and 8), in whichcase flapping displacement of a blade through the flapping rangerepresented by angle F, Figure 8, will change pitch from thatcorresponding to line I9 at the bottom of the chart Figure 7 to thatrepresented by line Tic, being the same amount of change at all radii.

Regardless of the specific location chosen for joint 58 an consequentspecific pattern obtained for line I1, if the axial component ofrelative air flow through the rotor is radically increased, as forinstance by giving a considerable tilt to the effective plane ofrotation of the rotor combined with considerable forward speed of thecraft, this increase of axial flow should be accompanied by acorresponding increase in the relative steepness of pitch toward theroots of the blades, if best eflioiency is to be maintained, whichrelative steepening may be introduced by a raising of spider 53 (Figures3 and 6). Thus any desired blending of the axial flow curves of Figure'7 (such as curves 69, SI and 86) with the translational flight curves(such as Tia, 11', 11b, 11c, and 11d) may be secured, to best suit anyparticular ratios of translational flight and axial flow velocities thatmay be encountered. Since introducing any large angles of tilt into therotor will require some steepening of blade pitch to maintain any givenflight angle, and since any further increase in blade pitch than thisamount will tend to steepen flight angle and further increase axialflow, it is apparent that the adjustments of spider 53 necessarilyincidental to flight control will automat cally take care of introducingalterations into the pattern of blade pitch distribution of such anature as to better adapt the blades to function with variousproportions of axial and translational velocit es, these alterationsbeing so related to the ratio of axial flow components that increase inthe ratio of axial flow components of velocity will cause the patternsof blade pitch distribution to become more like those hereinbeforedescribed as adapted to axial flow conditions, coinciding therewith whenthe controls are so set as to produce truly axial flow.

A preferred method of controlling the vertical position and tilt ofspider 53, so as to thereby control the pitch of the rotor blades bothsynchronously and cylically, is illustrated in Figures 3 and 5. As thereindicated spider 53 is fastened by means of bolts I5 to a floatingcylindrical frame 88. which is supported from sleeve 8| by means of auniversal or gimbal mounting including two pins 82 extending radiallyoutward from sleeve 84 and forming a pivotal mounting for ring 83, whichin turn carries two pins 84 at right angles to pins 82 and forming apivotal connection between ring 83 and frame 88. Sleeve 8| is splined toshaft 33 and vertically slidable on the shaft. The vertical position ofsleeve 8| and hence of frame 88 and spider 53 is controlled through abell crank 86, pivotally mounted to the frame of the craft at 81 andterminating in a fork including pins 88 which ride freely in an externalgroove of the outer race 89 of a ball bearing, the inner race 98 ofwhich is integral with sleeve 8 I. In order to prevent the outer ballrace 89 from being continually rotated by the friction of the ballbearing and possibly wearing flats on pins 88 a pin 9| is inserted inthe groove of ball race 89, thus limiting rotational displacement ofrace 89 relative to pins 88. It will be apparent that the angularposition of bell crank on its pivot 81 will determine the average heightof spider 53 and hence the average effective pitch of the rotor blades.

Tilting of spider 53 about one axis, fixed relative to the craft, willcontrol longitudinal movement of the craft, while tilting about an axisperpendicular thereto will control lateral movement. Arrangements aretherefore provided for independently controlling the tilt of frame 88,and hence of spider 53 integrally attached thereto, about two mutuallyperpendicular axes fixed relative to the craft. For this purpose, asshown in Figures 3 and 5, the inner race 93 of a ball bearing isintegrally attached to frame 88 while the outer race 94 thereof ispivotally connected by two pins 95 to a fork 96 which is seated insocket 91 on the end of link 98 so as to be freely rotatable about thelongitudinal axis of the link but not otherwise displaceable relativethereto. Link 98 is connected by means of a hinge pin 99 to a rod I88,which in turn is connected by a universal joint I8I to a shaft I82. RodI88 is surrounded in part by a sleeve I85 pivotally mounted on hingepins I86, the central axis of which passes through the center ofuniversal joint IN. The angular position of sleeve I85 on pins I86 iscontrolled by a link I81 pivotally attached to the sleeve at I88, andoperated by suitable manual controls (not shown).

Any longitudinal displacement of link I81 will therefore result indisplacement of the bottom of frame 88 in the same direction and by aproportional amount, resulting in a rocking of the frame on itsuniversal mounting, and consequent tilting of spider 53, about an axisperpendicular to the 75 plane of Figure 5, while rotation of shaft I82and consequently of rod I will result in displacement of the bottom offrame 80 in a direction substantially perpendicular to that controlledby link I07, and tilting of frame 80 and spider 53 substantially aboutan axis lying in the plane of Figure 5. Therefore the arrangement issuch that the two members 86 and 98 suffice to transmit to spider 53 allcontrol movements including vertical displacement, and tilting about allaxes.

The principle of utilizing pitch adjustment of an airscrew involvingintroduction of twist in accordance with a prearranged pattern, inconjunction with a properly coordinated blade width pattern, may beadvantageously employed in airscrews utilized for propulsion, such forinstance as airplane propellers. As the range of blade pitch adjustmentutilized in flight increases there is a corresponding increase in thesacrifice in efiiciency necessarily incident to the usual practice ofemploying a rigid blade in which the pattern of blade pitch distributioncannot be altered except by imparting identical increases or decreasesin pitch to all blade elements. With such a blade the builtin bladetwist is at best a compromise, and with increasing range of pitchadjustment a very serious compromise, between the patterns of pitchdistribution best suited to different stages of pitch adjustment. Theincreasing speeds and altitudes frequently encountered in flight israpidly increasing the seriousness of this problem.

At the same time the increased propeller disc loading, incidental to theuse of higher horsepowers per propeller is rapidly requiring increase inpropeller solidities, which if carried very far involves serioussacrifice in efficiency unless steps are taken to insure substantiallyequal axial velocity over the entire active portion of the propellerdisc area. As previously described this requires both the adoption of ablade width pattern, in which the width of the active portion of eachblade varies substantially in inverse proportion to the radius from thepropeller axis, and also the use of a flexible blade with controlledvariable twist to permit of suitable alteration of the blade pitchdistribution pattern as pitch changes, preferably so that the incrementsof pitch angle will also vary substantially inversely as the radius fromthe propeller axis.

Figures 10, 11, and 12 illustrate a propeller embodying theseimprovements. It will be noted that the central hub shielding I00 isvery much larger in diameter than is customary in present propellerpractice but that it approximates the lines IOI, Figure 10, whichrepresent the theoretical continuation inward of the outer part of theblade outline if the blade were to continue to broaden in inverse ratioto radius. Each blade comprises an outer substantially rigid tip I03which, as illustrated in Figure 11 is integral with a blade shaft I04rotatably supported (by means of a bearing capable of acting as a thrustbearing) in rim I06 of a hub structure I01 integral with propeller shaftI08. Shaft I04 is also guided within a blade base piece I I0 rotatablysupported in rim I and having an outer surface which blends with hubshielding I00 and preferably conforms to a sphere having theintersection of the axis of shafts I04 and I08 as its center. Theportion of each blade intermediate between base piece I I0 and tip I03comprises a hollow and relatively flexible shell I I I integrally joinedto a base socket piece I I2 and a tip socket piece I I3. These socketpieces are designed to respectively fit over tenon II4 of tip I03 andtenon II5 of base piece IIO, each of which tenons accurately fits itsmating socket and is of such shape as to prevent the tenon from rotatingwithin the socket. The shell III may, for instance, be made of moldedplastic, with socket pieces H2 and II3 constituting metal inserts moldedintegrally into the plastic piece. Alternatively shell III may be ofthin flexible metal welded to the two socket pieces.

As in the case of the rotor blade previously described, arrangements areprovided for rotating the base of the blade through an angleproportional to, but much greater than, the angle through which the tipis rotated, the difference in the two amounts of rotation being taken upin twist of shell I I I, which serves to effect a gradual transitionbetween the pitch setting of the base and that of the tip according to apattern dependent upon the stiffness pattern of the shell.

The construction of the propeller hub and the mechanism for controllingand altering blade pitch will now be described in detail. Propellershaft I08 is supported in a bearing III mounted in frame member IIB ofthe craft. Bearing II'I terminates forwardly in a gear I20 integral withthe bearing. Integrally attached to rim I05 of hub member I0? is aninternal gear I2I concentric with shaft I08. Pivotally mounted on thecentral hub I23 of hub member I0! is spider I24 including one or morearms I25, each carrying a stud I26 on Which are mounted two similar butindependently rotatable planetary pinions I21 and I28. Planetary pinionI21 meshes inwardly with gear I20, which therefore serves as the sungear of a planetary system, and which as previously described isintegral with the framework of the craft. This same planetary pinion I21meshes outwardly with internal gear I2I, which as previously describedis integral with propeller hub member I01 and shaft I03. Hence planetarypinion I21 will serve to definitely position stud I26 at all times,causing it to revolve about shaft I08 at a fixed fraction of the speedof revolution of the shaft.

Rotatably mounted on the central sleeve portion of bearing I I1 is acompound gear including at one end thereof gear portion I30, identicalin size and pitch with gear I20, and at the other end a gear portionI3I, which may be of any convenient diameter. Gear I30 acts as a sungear for planetary pinion I28, just as gear I20 does for pinion I21.Rotatably mounted on compound gear I30, I3I is a hub I32 carrying a discI33 integrally supporting a ring I34 the internal surface of whichconstitutes an internal gear of the same size and pitch as internal gearI2! previously described. The external surface of ring I34 constitutes aspur gear meshing with three pinions I35 (one for each blade). Eachpinion I35 is integral with a shaft I30 and a worm I31 meshing with aworm gear I38 which in turn is integral with the blade shaft I04previously described. Therefore any rotation of a gear I35, in onedirection or the other relative to the propeller hub, willproportionately increase or decrease the pitch of the correspondingpropeller tip I03.

Each pinion I35 is connected through idler I40 to a pinion I4I integralwith shaft E42 and worm I43, which in turn meshes with worm gear I44integral with base piece H0 of the related blade. Worm I43 is of so muchsteeper pitch than worm I31 as to turn base piece III] at the desiredmultiple of the amount that tip I03 is turned. As in the case of therotor blade the desired ratio of movement of tip I03 and base piece IIOcan be obtained in spite of torsional deflection of shaft I04, byestablishing the worm gear ratios in the proportion to each otherindicated by Equations 4 and 6. Also the worm I31 and worm gear I38 maybe dispensed with if the relative torsional stiffness of shaft I04 andshell III satisfy Equations 5 or 7.

The pitch of the blades is controlled from the craft in accordance withthe rotational setting of shaft I50, which extends out from the craftinto the vicinity of the propeller and terminates in a gear I5I meshingwith the gear I 3|, previously described. As long as shaft I50 remainsstationary, gear I5I prevents rotation of compound gear I3I, I30. Aslong as gear I30 is held stationary ring gear I34 will be revolved byplanetary pinion I28 at exactly the same speed as the propeller, sincein the parallel planetary unit which determines the movement of stud I26sun gear I is always stationary and internal gear l2I always turning atthe same speed as the propeller. With ring gear I34 thus fixed relativeto the propeller hub, gears I35 and MI will remain fixed relativethereto, and the pitch of the blades will remain constant. However, ifshaft I50 is rotated in one direction or the other gear I30 will becorrespondingly rotated, thus imparting an increment of rotation toplanetary pinion I20 which will advance or retard ring gear I34 relativeto the propeller hub, which movement will rotate all gears I35 and MIequally and impart an equal increase or decrease in pitch to allpropeller blades, but within each blade the amount of pitch change willgradually increase toward the root in accordance with a patternprearranged to permit each element of the blade to be set at the pitchangle bearing the best relation to the pitch angles of the otherportions of the blade. This pattern may preferably correspondsubstantially to that represented by curves 08, 02, and 06 of Figure 7applying to axial flow conditions of the sustaining rotor, as previouslydescribed.

My copending application, Serial Number 488,917, filed May 28, 1943discloses how, by surrounding a propeller by a properly shaped duct, thestatic pressures developed by the propeller may be greatly increased,and the range of pitch angles that can be advantageously employed may beconsiderably increased, thus rendering propellers effective atunprecedented speeds and altitudes. However, if rigid blades areemployed instead of flexible blades with adjustable blade twist inaccordance with the present invention, the sacrifices in efiiciency andperformance encountered by the necessity of employing unsuitable pitchangles on some portions of the blades are particularly large because thestatic pressures handled are so large that inequalities therein may haveserious effect. The unusual advantages of the ducted propeller cantherefore only be realized in anything approximating their fullpotential measure, by utilizing in the ducted propeller, blades ofadjustable twist together with means for properly controlling andadjusting the twist during operation. The application of the presentinvention to the ducted propeller therefore constitutes a most importantpossible field for its use and one making possible the attaining ofresults which would not be possible except through combining in a singlepropulsion unit the properly shaped duct and propeller blades havingtwist adjustable in accordance with the proper pattern of pitchdistribution. As set forth in my above mentioned copending applicationSerial Number 488,917 the duct, in order is to operate to the bestadvantage is preferably one proportioned inaccordance with the formula:

20,000H a re We e in which:

i=the ratio of the net area of duct inlet to the gross disc area sweptby the airscrew,

e=the ratio of the net area of duct outlet to the gross disc area sweptby the airscrew,

H=horsepower tobe delivered to airscrew under the specific or averageconditions selected as those at which it is desired maximum operatingefficiency shall occur,

V=craft velocity in miles per hour under these same conditions,

a=density ratio at. altitude for these same conditions; that is,ratio ofstandard air density at the specific or average altitude selected asthat at which it is desired maximum operating efiiciency shall occur tostandard air density at sea level,

D=diameter of airscrew, in feet,

S=so1idity ratio of net airscrew area; that is the ratio that the areaof the projection of the airscrew onto a plane perpendicular to theairscrew axis minus the hub area which constitutes that portion of theswept disc area not available for airflow bears to the gross swept discarea of the airscrew minus the same hub area.

A propeller constructed in accordance with the present invention, whenplaced within a duct proportionedin accordance with the foregoingformula, will make it possible to attain unprecedented increases inthrust values at high efiiciencies, for such a result is contingent upondeveloping static pressures which can only be maintained if all parts ofthe blades are capable of developing pressure in a fairly equitabledegree, to do which over any large range of blade pitch settingsrequires a construction such as contemplated by my present invention,and requires that the propeller be surrounded by a duct to preserve thepressure, and that the duct be so shaped as to permit of the eflicientconversion of the pressure into thrust under the actual flightconditions which will be true if the duct shape substantially accordswith the above formula.

Figure 13 illustrates such an embodiment, in which the propeller has ahub covering I00 and blades comprising tips I03, socket pieces H2 andH3, flexible shells III, and base pieces 0', all substantiallycorresponding to the respective parts I00, I03, H2, H3, III, and H0, ofFigures 10, 11, and 12. The propeller of Figure 13 also comprises themechanism illustrated in Figure 11 and described above for adjustingblade pitch. This propeller is placed within a duct I60, preferablyconstructed in accordance with the disclosure of my copendingapplication Serial Number 488,917, and supported from a fixed portion ofthe craft by means of vanes I6l which serve also as straightening vanesto neutralize the rotational components of the slip stream.

' I claim:

1. In an airscrew having a rotatable hub, the combination of one or moreflexible blades; the root of each blade being adjustably attached tosaid hub for angular adjustment about an axis extending outwardly fromsaid hub; and the width of each blade varying in substantially inverseproportion to" the radius from the center 19 of the airscrew, means foreffecting angular adjustment about the blade axis of a section of eachblade adjacent the root thereof, and means for preventing equal angulardisplacement about said blade axis of a section of said blade outwardlyspaced from the root thereof.

2 In an airscrew having a rotatable hub, the combination of one or moreblades; the root of each blade being adjustably attached to said hub forangular adjustment about an axis extending outwardly from said hub; thewidth of each blade varying in substantially inverse proportion to theradius from the center of the airscrew; and each of said blades having aflexible portion, means for efiecting angular displacement about theblade axis of a section of each of said flexible portions nearest theblade root, and means for preventing equal angular displacement aboutthe blade axis of a section of each of said flexible portions remotefrom the blade root; the efiective polar moment of inertia of theflexible portion of each of said blades varying throughout its lengthsubstantially in proportion to the square of the distance from hubcenter.

3. In an airscrew having a rotatable hub, the combination of one or moreflexible blades, the root of each blade being adjustably attached tosaid hub for angular adjustment about an axis extending outwardly fromsaid hub; and the width of each blade varying in substantially inverseproportion to the radius from the center of the airscrew, means foreffecting angular adjustment about the blade axis of a section of eachblade adjacent the root thereof, and separate means for efiectingangular displacement about said blade axis of a section of said bladeradially spaced from the root thereof.

4. In an airscrew having a rotatable hub, the combination of one or moreblades; the root of each blade being adjustably attached to said hub forangular adjustment about an axis extending outwardly from said hub; thewidth of each blade varying in substantially inverse proportion to theradius from the center of the airscrew;

and each of said blades having a flexible portion, means for effectingangular displacement about the blade axis of a section of each of saidflexible portions nearest the blade root, and separate means forefiecting angular displacement about said blade axis of a section ofeach of said flexible portions remote from the blade root.

5. In an airscrew having a rotatable hub and a blade of length Rmeasured from hub center the root of which blade is adjustably attachedto said hub for angular adjustment about a blade axis extendingoutwardly therefrom; the combination of means for efi'ecting an ular dislacement about the blade axis of a cross-section of the blade adjacentthe root thereof andlocated at the distance 12 from the center of thehub, and means for preventing equal angular displacement of across-section of the blade remote from the hub and located at thedistance 11 from the center of the hub, said last mentioned meanscomprising a member connecting said hub to said blade in the vicinity ofsaid last mentioned crosssection thereof, the torsional stiffness of theportion of the blade lying between said two crosssections beingsubstantially 6. In an airscrew having a rotatable hub and a bladeattached thereto and extending outwardly a distance R from the center ofthe hub; the combination of means for effecting angular displacementabout the longitudinal blade axis of a cross-section of the bladeadjacent the root thereof and located at the distance m from the centerof the hub, means for effecting a difierent angular displacement aboutsaid blade axis of a cross-section of said blade remote from the rootthereof and located at the distance n from the center of the hub, saidlatter means comprising an angularly displaceable member having atorsional stiffness l/f, as great as that of the portion of th bladeintermediate between said two cross-sections, said member extendinglongitudinally ofthe blade from the vicinity of said first mentionedcross-section to the vicinity of said second mentioned cross-section,and mechanism connecting said blade in the vicinity of said firstmentioned cross-section to an adjacent portion of said member tomaintain the angular displacement of said portion of said memberproportional to the angular displacement of said cross-section of saidblade substantially in the ratio:

'7. In an airscrew having a rotatable hub, and a blade having a flexibleportion and a blade root which is adjustably attached to said hub forangular adjustment about a blade axis extending outwardly from said hub;the combination of means for effecting angular displacement about theblade axis of a cross-section of the blade adjacent the root thereof,and means for preventing equal angular displacement of a cross-sectionof the blade remote from the hub; the efiective polar moment of inertiaof the flexible portion of the blade varying throughout its lengthsubstantially in proportion to the square of the distance from hubcenter.

8. In an airscrew having a rotatable hub, and a blade connected theretoand including an outer aerodynamic shell; the combination of means forchanging the pitch of a first cross-section of said blade at a positionremote from the hub in response to flapping displacement of the bladerelative to the hub, said means comprising a member of appreciabletorsional stiffness integrally attached to said shell in the vicinity ofsaid crosssection and hingedly attached to said hub by means of aflapping hinge making an acute angle with the longitudinal axis of theblade, and means for changing the pitch of a second cross-section ofsaid blade at a position closer to the hub than said first cross-sectionin response to the same flapping displacement of the blade, said lastmentioned means comprising a pitch changing hinge pivotally attachingsaid shell in the vicinity of said second cross-section to said memberso that said member will vertically displace a portion of said shell atsaid second cross-section whenever said member is angularly displacedupon said flapping hinge, and means for restraining vertical movement ofa portion of said shell in the vicinity of said second cross-section.

9. An airscrew of the character set forth in claim 8 wherein therestraining means comprises a member adjustably attached to the shell ofthe blade.

10. In an airscrew having a rotatable hub and 21 a blade attachedthereto; means "for cyclically varying the pattern of pitch distributionalong the blade, comprising means responsive to nonaxial components ofairflow for causing cyclic pitch changes in a first portion of theblade, a tiltable swash plate, means connecting a second portion of theblade to the swash plate for causing cyclic pitch changes in said secondportion of the blade different from those in the first portion thereof,and a torsionally flexible aerodynamic shell connecting said firstportion and said second portion.

HAROLD T. AVERY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Number 272,578 541,320 851,766

Name Date Pescara Feb. 10, 1925 Bushyager Dec. 21, 1926 Roberts June 19,1934 Breguet Jan. 1, 1935 Flettner Feb. 11, 1936 Kelly Sept. 3, 1940Muller-Kenth et a1.

Sept. 29, 1942 McElroy et al Oct. 13, 1942 FOREIGN PATENTS Country DateGreat Britain June 9, 1927 France July 26, 1922 France Jan. 15, 1940

